It is well-known that InP-based opto-electronic devices, such as distributed feedback (DFB) laser, distributed bragg reflector (DBR) laser, Fabry-Perot (FP) laser, waveguide, optical switch, amplifier, modulator and integrated devices etc., play an important role in optical fiber communication systems. Optical fiber communication systems require high performance lasers which are characterized by low threshold, high linear power output, stable fundamental mode, high modulation response, high temperature operation, etc.
There are two common types InP based lasers used for opto-communication systems. The first type is weak index-guided laser, which requires that the thickness of at least one layer be laterally non-uniform. An example is ridge waveguide (RWG) laser, where the cladding layer is etched into ridge type, and the active layer laterally extends beyond ridge. the optical confinement is formed by the index step (.about.10.sup.-2) by both ridge waveguide geometry effective index step and the carrier induced index change. But the carriers are still not confined laterally. These type of lasers can be easily fabricated, for the FP lasers only require one epitaxy growth for DFB lasers it only require two types of lasers. But due to the intrinsic mechanism of weak index guided laser, in some functions, such as stable fundamental mode controlling, they are not good enough.
The second type of laser is the strongly index-guided laser where the active layer is surrounded by a larger band gap, low index material (the index step is about 0.2) which forms a carrier and optical confinement, such as buried hetero-structure (BH) with reverse P-N junction lasers, The InGaAsP active strip is buried with a larger band gap InP material, and so the regrown material block current flows through reverse-biased junctions. So this type lasers combines all the good features of current, carrier, and photon confinements, which could satisfy the most demanding application in the fiber optical communication systems.
However, the conventional fabrication process of such BH InP-based opto-electronic devices is very complicated. Without loss of generality, we will focus on the DFB laser for the following introduction because BH DFB laser is the most important device and is also the most difficult device to fabricate. Other BH InP-based devices can be similarly fabricated without grating. FIG. 1 illustrates a typical buried hetero-structure DFB laser processing procedure, which comprises the following steps: (1) FIG. 1a: first growth of buffer layer, cladding layers and active layer; (2) FIG. 1b: fabrication of grating and (3) FIG. 1c: regrowth of cladding layer; (4) FIG. 1d: deposition of dielectric film such as SiO.sub.2 or Si.sub.3 N.sub.4 film and wet etching above layers into specific strip rib; (5) FIG. 1e: selective regrowth of p-n current block layer; (6) FIG. 1f: removal of dielectric film and regrowth of the cladding and contact layer; (7) FIG. 1g: wet etching to form two grooves parallel to the first rib to reduce the device capacitance; (8) FIG. 1h: deposition of dielectric layers and etching out a window for the contact layer and finally, evaporation of contact metals for both n and p metallisation.
The complexity of the regrowth, selective regrowth, cleaning, deposition and removal of dielectric film, and wet etching of the stripe mesa, greatly increases the cost of BH DFB lasers and reduces the yield.
Moreover, as can be seen in FIG. 2(a), for this type of BH laser, there are unavoidable leakage currents both from I to II and from III to IV for the pnpn type thyristor. From the equivalent circuit FIG. 2(b), the anode current I.sub.A until the thyristor break over could be: EQU I.sub.A =[Io+(.alpha..sub.2.cndot.M.sub.n.box-solid.I.sub.g)]/[1-(.alpha..sub. 1.cndot.M.sub.p +.alpha..sub.2.cndot.M.sub.n)] (1)
Where
I.sub.o : a leakage current of the junction J.sub.2 PA1 I.sub.g : gate current PA1 .alpha..sub.1 : common based current gain of transistor T.sub.r1 PA1 .alpha..sub.2 : common based current gain of transistor T.sub.r2 PA1 M.sub.n : avalanche amplification factor for the electrons in the depletion layer of junction J.sub.2 PA1 M.sub.p : avalanche amplification factor for the holes in the depletion layer of junction J.sub.2.
As can be seen from the equation (1), the break-over conditions of the thyristor are represented by the following equation (2). EQU .alpha..sub.1.cndot.M.sub.p +.alpha..sub.2.cndot.M.sub.n =1 (2)
The .alpha..sub.1 and .alpha..sub.2 of the equation (2) are drastically increased normally when the anode current I.sub.A increases or the temperature of the diode rises.
Further, the M.sub.n and M.sub.p generally have the dependency as represented by the following equation (3). In the equation (3), in the case of V&lt;&lt;V.sub.a1 M=1 is satisfied, but as the V approaches V.sub.a, the M is drastically increased. EQU M=1/[1-(V/V.sub.a).sup.2 ] (3)
Because of such behaviors of .alpha. and the M together with the relationship of the equation (1), increases in the gate current and the applied voltage V.sub.a, and temperature rise cause an increase in the anode current of the thyristor. Further, since a positive feedback system of increasing the .alpha..sub.1, and .alpha..sub.2 due to the increase in the anode current is formed, the thyristor feasibly break over. So the leakage current under high temperature and high output power operation is unavoidable for the p-InP/n-InP reversed biased junction BH laser.
To address the above mentioned problems, we propose to replace InP p-n reverse junction block layer in conventional buried heterostructure (BH) InGaAsP/InP lasers by Al-bearing compound native oxide layer. This type of laser device not only keeps good features of the conventional BH laser by more facilitated processing method, but also the high insulation characteristic of native oxide will avoid the leakage current from p-n-p-n InP thyristor like junction dependence on the temperature and high power operation, so it will improve the high temperature and high power performance of the conventional BH InGaAsP laser.
There has been an interest in the recent years to apply Al-bearing compound native oxide layer to opto-electronic devices because it is an insulator layer and has a low refractive index (n.about.1.6). The native oxide of Al-bearing compounds provides both electrical and optical confinement and, moreover, simplifies processing of the lasers. The principle of native oxidation is to expose heated Al-bearing materials in water vapor saturated gaseous ambient to form an anhydrous oxide of Aluminum which is very stable and does not degrade under normal operating conditions. This thickness of this type of oxide layer is the same or less than the thickness of the as-grown Al-bearing materials and it does not cause disruption or induce strain. Moreover, the refractive index is less than 2, and therefore the oxidized layer will facilitate electron block and optical confinement.
For the surface-emitting lasers, AlAs oxide has been successfully used for DBR structures and for current constriction to achieve low threshold devices. For edge emitting lasers, the native oxide of AlGaAs has been utilized to fabricate stripe-geometry lasers and index-guided buried ridge waveguide lasers. This technology is widely used for GaAs-based optoelectronic devices containing Al-bearing materials such as AlAs, AlGaAs, AllnP etc.. See U.S. Pat. Nos. 5,262,360 by N. Holonyak, Jr and J. Dadallesasse (AlGaAs native oxide), 5,550,081 by N. Holonyak, Jr., S. A. Maranowski., method of semiconductor device by oxidizing Aluminum-bearing III-V semiconductor in water vapor environment), S. A. Maranowski, A. R. Sugg, E. I. Chen, and N. Holonyak, Jr., Appl. Phys.Lett. 63, 1660 (1993), Y. Cheng, P. D. Dapkus, M. H. MacDouugal, and G. M. Yang, IEEE Photonics Technol, Lett. 8, 176 (1996), J. J. Wierer, S. A. Maranowski, N. Holonyak, Jr., P. W. Evans, and E. I. Chen, Appi, Phys. Lett. 8, 176 (1996), D. L. Huffaker, D. G. Deppe, Kumar, and T. J. Rogers, Appi. Phys. Lett. 65. 97 (1994), K. L. lear, K. D. Choquette, R. P. Schneider, Jr., S. Kiloyne, and K. M. Geib, Electron. Lett. 31, 208 (1995), B. J. Thibeault, E. R. Hegblom, P. D. Floyd, R. L. Naone, Y. Akulova, and L. A. Coldren, IEEE Photonics Technol. Lett.8, 593 (1996), P. D. Floyd, B. J. Thibeault, E. R. Hegblom, J. Ko, L. A. Coldren, and J. L. Merz, IEEE Photonics Technol, Lett. 8, 590 (1996).
So far, advances in the Al-bearing native oxidation in InP-based devices have been much slower. The native oxide of InAIAs, lattice-matched to InP, has been employed in the long wavelength InP/InGaAsP gain-guided or weak index guided buried ridge waveguide lasers, and as a gate insulator in a InAlAs/InGaAs MOSFET. There has also been a report using native oxide of AlAsSb, lattice-matched to InP with Al mole fraction of 1.0.
Here, we propose to apply Al-bearing oxide to InP-based BH lasers to facilitate conventional InP based BH laser processing and prevent leakage current for the conventional BH laser.